Perhaps I posed the question incorrectly and it should have been "in total"
I think they may have classed double faults as eight strikes of the ball, but I'm certain in tennis if you attempt to hit the ball on a serve but miss it, it is classed as a fault, so zero would still be correct
However, the contestant answered 8 (serve and a winning return) and it was correct
The next day they said they'd made an error and the answer should have been 4 ( 4 aces) but they allowed the contestant to keep the money he'd won